This invention relates generally to thermal transfer printer and, more particularly, it is concerned with a printer of the type described which is suitable for use in receiving electric picture signals and reproducing on a sheet, such as paper, images which correspond to the picture signals received.
One type of thermal transfer printer known in the art is disclosed in a report entitled "COLOR VIDEO PICTURE PRINTER" by Masuda, which appeared in "IEEE TRANSACTION ON CONSUMER ELECTRONICS", vol. CE-28, August 1982. In this printer, a recording paper or sheet is affixed at one end thereof to the outer circumferential surface of a platen of substantially cylindrical configuration and is wound thereon. Then, the platen is driven for rotation and, while it makes one complete revolution, a thermal head is pressed against the recording sheet through a sheet of ink support material generally referred to as a color dye sheet. The color dye sheet is heated by the thermal head and an image is printed on a recording sheet as the ink of the color dye sheet is transferred to the recording sheet. There are three types of color dye sheet including those of cyan, magenta and yellow, for example. After the transfer printing using one type of color dye sheet is finished, the same process is repeated by using a color dye sheet of another color, to print an image of another color on the recording sheet while the platen makes another complete revolution. Thus, by supposing a plurality of images of different colors one over another on the same recording sheet, a desired image can be printed by transfer printing on the same recording sheet in desired colors.
The printer of the prior art of the aforesaid construction offers the advantage that, since the recording sheet is fixedly secured to the platen of substantially cylindrical configuration, no variation or shift occurs in the position of the recording sheet as the platen rotates, so that the risk that irregularities might be caused to occur in the colors of the printed image by the mismatching of the recording sheet with the color dye sheets. However, when the recording sheet used is large in size, it would be necessary to increase the diameter of the platen to enable the recording sheet of large size to be wound thereon completely. Besides, the platen has at one location on its outer circumferential surface a mechanism for affixing one end of the recording sheet. When it is desired to print images in a single color continuously on a web of paper in the form of a roll, for example, by using this platen, the fixing mechanism referred to hereinabove interferes with the operation of winding the web on the platen, making it impossible to achieve correct positioning of the web on the platen.
Another printer disclosed in Japanese Patent Application Laid-Open No. 58-140271, for example, is known as another type of this apparatus. In this printer, a roller is mounted on either sides of a platen in a manner to hold the platen by two rollers. In operation, one recording sheet is inserted between the platen and each roller and wound on about one-half the outer circumferential surface of the platen. Thus, the recording sheet is brought into intimate contact with about one-half the outer circumferential surface of the platen because it is forced thereagainst by the two rollers. As the platen is rotated in a predetermined direction, the two rollers rotate together with the platen, so as to feed the recording sheet on the platen in one direction. At this time, a thermal head is urged against the recording sheet on the platen through an ink donor film. An electric current is successively passed to the thermal head in accordance with image information to heat the ink donor film which is positioned against the thermal head, so that ink adheres to the recording sheet. Rotation of the platen feeds the recording sheet, to successively record images starting with the location on the recording sheet at which recording is initiated. As the thermal head reaches a location on the recording sheet at which recording is terminated, the platen is rotated in the reverse direction, to feed the recording sheet in the reverse direction and return the same to the recording initiation location. While the recording sheet is being returned to the recording initiation position, the ink donor film is moved so that a different color zone is brought to the recording initiating position. The ink donor films each have a plurality of zones or bands of different colors arranged lengthwise thereof. After the used ink donor film has been replaced by a new ink donor film, the aforesaid process is repeated to record image information on the recording sheet, starting at the recording initiation location. In this way, recording of image information in color is effected by superposing a plurality of color images one over another on the same recording sheet.
This printer is capable of recording image information on a recording sheet of large size and on a continuous web of paper in roll form. However, when recording is effected by superposing one color image portion over other color image portions, it is necessary to feed the recording sheet in the reverse direction when replacement of the color zone on the ink donor sheet to another color zone is effected, to return the recording sheet to the recording initiation location. This operation is time-consuming, thereby increasing the overall period of time required for recording image information. Another disadvantage of this printer is that, since the recording sheet is moved back and forth a plurality of times by rotating the platen in opposite directions, changes or shifts might occur, although slight, in the position of the recording sheet on the platen as the recording sheet is moved in reciprocatory movement several times, thereby causing inaccurate overlapping to occur in the colors of the recorded image.
Another printer of this type of the prior art is disclosed in Japanese patent application Laid-Open No. 57-45072. This printer has two rollers arranged parallel to each other in spaced-apart relation and connected together by an endless belt. In operation, a recording sheet is affixed at one end thereof to the surface of the belt and is wound thereon. Then, one of the rollers is rotated to cause the belt having the recording sheet wound thereon to move on the two rollers. While the belt makes one complete revolution along the path of its travel on the two rollers, a thermal head presses an ink donor sheet against the surface of the recording sheet wound on the surface of the belt, and the ink donor sheet is heated by the thermal head to record image information on the surface of the recording sheet. A plurality of ink donor sheets are used, and the same process is repeated to provide a sheet of recorded image by superposing image information of different colors one over another.
Some disadvantages are associated with this printer. The belt is formed of elastic material and tends to expand and contract (when the belt is moved in contact with two rollers, it is expanded; when the belt is moved out of contact with the rollers, it is not expanded). Meanwhile the recording sheet neither expands nor contracts. Therefore, when one end of the recording sheet is affixed to the surface of the belt, the recording sheet might separate itself from the surface of the belt and become loose during the movement of the belt, so that the recording sheet might catch against other parts of the printer and become wrinkled. Also, if a slip occurs between the belt and the rollers, the position of the recording sheet wound on the belt might undergo a change and the sheet of recorded image might suffer inaccurate overlapping of color. In this printer, no attention has ever been paid to these problems.
The printers of the prior art outlined hereinabove are summarized as follows. In apparatus for recording an image in color on a recording sheet by superposing a plurality of colors of image information one over another, the problems that should be solved are that (1) no inaccurate overlapping of color should occur in the sheet of recorded image, (2) the apparatus should be able to handle recording sheets of a wide range of sizes, (3) the apparatus should be compact in overall size, and (4) the apparatus should be high in performance and free from a loss of time in feeding sheets.